Helma Dik just made an announcement on her website about a online interface for using digitized Greek lexicons from Perseus:

Helma Dik wrote:This summer, we launched Logeion, an integrated interface for looking up entries in the dictionaries and reference works from the Perseus collection. Logeion is inspired by DVLF, the Dictionnaire vivant de la langue française, but re-designed from the ground up by two Chicago Classics undergraduates, Josh Goldenberg (class of 2012) and Matt Shanahan (class of 2014). Some more details: Logeion does not allow full text search - for this you should still use the separate search pages for the dictionaries, or the homesites of the DuCange and Latin-Dutch dictionaries (see Logeion's About page).

I've been playing with it and its extremely easy to use, providing all the available Perseus lexical entries as well as a list of example sentences pulled directly from the Perseus corpus:

If you use Chrome, be sure to read through her whole post because there's a pretty neat feature for integrating Logeion directly into the browser. I'm sure it would be possible to write a Firefox plugin that does the say...though that's beyond my technical abilities.